In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called “NodeB”. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs).
In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity.
Multimedia Broadcast Multicast Service (MBMS) is a broadcasting service that can be offered via existing GSM and UMTS cellular networks. MBMS uses multicast distribution in the core network instead of point-to-point links for each end device. MBMS enables cellular networks to distribute, by shared broadcasts, multimedia in a digital form, sent out simultaneously to all cell phone users within each transmitter footprint by a single shared transmission. MBMS has been standardized in various groups of 3GPP, and the first phase standards are found in UMTS release 6.
MBMS over single frequency networks (MBSFN) has recently been specified in 3GPP for Release-7 UTRA systems. MBSFN Release 7 has further optimized MBMS to boost transmission efficiency beyond what could be achieved with multicell MBMS transmission in Release 6. Multicast/broadcast single-frequency network (MBSFN) operation involves for simultaneous transmission of the exact same waveform from multiple cells. In this way the wireless terminal (UE) receiver perceives the multiple MBSFN cells as one large cell. Also, instead of inter-cell interference from neighboring cell transmissions, the wireless terminal experiences constructive superposition of the signals transmitted from multiple MBSFN cells. Advanced UE receiver techniques such as G-RAKE eliminate intra-cell interference by resolving the time difference of multipath propagation. The result is highly efficient radio broadcast transmission derived from WCDMA technology. An enhancement that eliminates inter-cell interference is the use of a common scrambling code on downlink carriers reserved for MBSFN transmission. Broadcast data is transmitted using the same logical and physical channel structures as for MBMS—that is, MTCH and S-CCPCH, along with control channels such as MCCH, MICH and MSCH. MBSFN improves power efficiency so much that the limiting factor in the radio downlink is no longer power but rather codes. 16 QAM modulation has been introduced for MBSFN to make good use of available radio resources. To significantly reduce battery consumption in UEs, one may even multiplex services per transmission time interval (TTI). See, e.g., Bergman, Johan, et al., “HSPA Evolution—Boosting the Performance of Mobile Broadband Access”, Ericsson Review No. 1, 2008, pg. 32-37.
MBSFN thus provides significantly higher spectral efficiency compared to Release-6 MBMS and is primarily intended for broadcasting high bit rate demanding Mobile TV services on dedicated MBMS carriers. Since broadcast only, MBSFN inherently target transmissions in unpaired frequency bands.
In single frequency network (SFN) transmissions, multiple base stations transmit the same waveform at the same time such that a terminal receives all base stations as if it were one large cell. For UTRA systems, SFN transmission implies that a cluster of time synchronized NodeBs transmit same contents using same channelization and scrambling codes. SFN transmission is illustrated in FIG. 1, where a terminal receives from two base stations. When using cell-specific scrambling, transmissions from the right hand side base station would represent inter-cell interference for the terminal in the adjacent cell. In a single frequency network, however, inter-cell interference become visible as additional multipath which can be taken into account by the terminal receiver as desired signal, resulting in considerable improved coverage.
MBSFN enhances the Release-6 MBMS physical layers by supporting SFN operations for MBMS point-to-multipoint (ptm) transmissions on a dedicated MBMS carrier. It also supports higher service bit rates and efficient time division multiplexing of services for reducing terminal battery consumptions by allowing discontinuous reception (DRX) of services. MBSFN use the same type of channels as used for Release-6 MBMS ptm transmissions.
In order to provide smooth integration of the MBSFN feature to any existing UTRA system, MBSFN has been specified for both FDD and TDD based physical layer downlink (DL) channel structures and thus encompasses MBSFN based on WCDMA (FDD); MBSFN based on TD-SCDMA (TDD); and MBSFN based on TD-CDMA (TDD). The FDD related MBSFN uses the WCDMA DL physical layer channels for transmission of data, and no paired uplink transmissions occur. In the TDD related MBSFN, all slots are used for downlink transmissions when networks are optimized for broadcast. Hence, no duplex occurs in MBSFN and the differences between FDD and TDD based MBSFN then mainly refer to the physical layer slot formats, the way Mobile TV services are time multiplexed and the chip rates in the case of the TDD options TD-SCMA and 7.68 Mcps TD-CDMA. (The chip rate for the third TDD option, 3.84 Mcps TD-CDMA, is the same as used in FDD.)
When transmitting in all downlink slots only, the meaning of TDD and FDD becomes obsolete in the sense that no duplex occurs in broadcast. As mentioned above, the difference then basically refers to the construction of the downlink physical channels. Therefore document RP-081124, Work item description of 3.84 Mcps TDD MBSFN Integrated Mobile Broadcast (incorporated herein by reference), specifies the WCDMA based MBSFN approach as a fourth TDD option in which all slots are dedicated for broadcast. This fourth TDD option has been referred to as MBSFN Integrated Mobile Broadcast (IMB) and has been targeted for 3GPP Release-8 UTRA systems. The MBSFN IMB fulfils relevant TDD RF requirements.
Digital modulation schemes are employed to determine how bits are mapped to the phase and amplitude of transmitted signals. Each consecutive bit sequence is mapped to a modulation symbol whose phase and amplitude correspond to one of the plural possible constellation points. For different modulation schemes the number of bits conveyed per modulation symbol is as follows: 1 for BPSK, 2 for QPSK, 4 for 16QAM, and 6 for 64QAM. Therefore, higher modulation order means greater achievable peak data bit rate for a given symbol rate. HSPA (3GPP Release 6) supports the QPSK and 16QAM modulation schemes in the downlink and the BPSK and QPSK modulation schemes in the uplink. Both MBSFN Release 7 and MBSFN IMB introduce higher-order modulations that increase the spectral efficiency.
The MBSFN Integrated Mobile Broadcast (IMB) transport channel baseband processing has the possibility to also map data on the Secondary Common Control Physical Channel (S-CCPCH) using an 16QAM signal point constellation, in addition to the ordinary QPSK. In contrast to HSDPA using 16QAM, discontinuous transmission (DTX) indication bits (alternatively referred to as discontinuous transmission (DTX) bits or DTX bits below) are used to fill up the S-CCPCH radio frames, as described in 3GPP TS25.212 “Multiplexing and channel coding (FDD)” v8.3.0, incorporated by reference herein.
In the case of QPSK, two bits (i1,q1) are mapped to one of four symbols as illustrated in FIG. 2. In the QPSK mapping of FIG. 2 the letter “A” represents the amplitude of the signals transmitted on the I and Q branches. When one of the bits is a DTX bit, only one of the I and Q signals is transmitted, or when both (i1,q1) are DTX bits no signals are transmitted.
FIG. 3 represents a case of 16QAM modulation wherein the quadruple (i1,q1, i2,q2) is to be mapped to one of 16 symbols. In FIG. 3 the notations “A1” and “A2” represent the possible amplitudes of the signals transmitted on the I and Q branches. For the 16QAM mapping of FIG. 3 the amplitude levels correspond to A1=0.4472 and A2=1.3416, as understood from 3GPP TS25.213 “Spreading and modulation (FDD)” v.8.2.0, which is incorporated herein by reference.
In contrast to the case of QPSK, the 16 QAM mapping of the four bits where one or several bits are DTX bits may not be well defined. For example, the quadruple (1,x,1,1) could be interpreted that the Q branch transmit a signal with the amplitude A2 (x=0) or −A2 (x=1). Thus, with DTX bits there is no one-to-one mapping between bits and symbols of a 16QAM signal point constellation. This makes expression of a discontinuous transmission (DTX) bit problematic for 16 QAM modulation. Table 1 shows possible combinations of discontinuous transmission (DTX) bits in a quadruple of four bits (“X” denoting the DTX bit and “B” denoting a non-DTX bit).
TABLE 1Combinations of DTX bits in a quadruple of 4 bitsi1i2q1q21XXXX2BXXX3XBXX4XXBX5XXXB6BBXX7BXBX8XBBX9BXXB10XBXB11XXBB12BBBX13BBXB14BXBB15XBBB